The present invention relates to a Cr- and W-containing low-alloy heat-resistant steel. More particularly, it relates to such a low-alloy steel which exhibits high creep strength at high temperatures above 550 .degree. C. and improved low-temperature toughness at room temperature or below and which is suitable for use as forgings and castings in various forms including heat-exchanger tubes, piping, heat-resistant valves, and connecting joints in applications such as boilers, chemical plants, and nuclear facilities.
Heat- and pressure-resisting parts for boilers, chemical plants, or nuclear facilities are usually made of a steel selected from austenitic stainless steels, high-Cr ferritic steels having a Cr content of 9%-12% (all percents given herein are by weight as long as they are concerned with an alloy composition), Cr--Mo low-alloy steels having a Cr content of up to 3.5%, or carbon steels. The material to be employed is selected by considering the environment in which it is used (including the temperature and pressure) and its cost.
Among the above-mentioned steels, Cr--Mo low-alloy steels containing up to 3.5% Cr are characterized in that they have improved oxidation resistance, hot corrosion resistance, and high-temperature strength compared to carbon steels. Their advantages over austenitic stainless steels are that they are significantly less expensive, have a lower coefficient of thermal expansion, and do not cause stress-corrosion cracking. When compared to high-Cr ferritic steels, they are less expensive and have better toughness, thermal conductivity, and weldability.
Typical examples of these low-alloy steels for tubes are T22 (2.1/4Cr-1Mo steel), T12, and T2, as defined in ASTM and ASME. These are generally called Cr--Mo steels. Many attempts to improve the high-temperature strength of these alloys by adding one or more precipitation-strengthening elements such as V, Nb, Ti, Ta, and B had been made. See, for example, Japanese Patent Applications Laid-Open Nos. 57-131349(1982), 57-131350 (1982), 62-54062(1987), 63-62848(1988), and 64-68451(1989).
Among the steels well known as a material for turbines is 1Cr-1Mo-0.25V steel, while 2.1/4Cr-1Mo- Nb steel was developed as a material for fast breeder reactors.
However, compared to high-Cr ferritic steels and austenitic stainless steels, these Cr--Mo low-alloy steels are still inferior with respect to resistance to oxidation and corrosion at high temperatures, and their high-temperature strength is significantly lower. Therefore, they suffer from problems when used at a temperature above 550.degree. C. In this respect, one of the present inventors has proposed a heat-resistant low-Cr steel which has improved resistance to oxidation and corrosion at high temperatures and improved high-temperature strength and which can be used as a substitute for high-Cr ferritic steels and austenitic stainless steels [Japanese Patent Applications Laid-Open Nos. 2-217438 (1990) and 2-217439(1990)].
The resistance to oxidation and to hot corrosion of a steel mainly depends on its Cr content. Therefore, an increased Cr content is effective in improving these properties. However, an increased Cr content also leads to a loss of the good thermal conductivity, toughness, weldability, and inexpensiveness which are characteristic of low-alloy steels. Of course, when low-alloy steels are used in an environment in which oxidation resistance and hot corrosion resistance are not critical, there is no need to increase the Cr content.
However, high-temperature strength is quite important in designing pressure-resisting parts and it is always desirable that the material have good high-temperature strength, regardless of the temperature at which it is used. Particularly, in heat- and pressure-resistant steel tubes used in boilers, chemical plants, and nuclear facilities, the wall thickness of the tubes is determined depending on the high-temperature strength of the steel.
Thus, the following advantages will be attained by improving the strength, and particularly high-temperature strength, of low-alloy steels.
(1) It becomes possible to use a low-alloy steel in those environments where corrosion is not so severe at high temperatures but where conventionally austenitic stainless steel or high-Cr ferritic steel has been used to assure high-temperature strength. The use of low-alloy steels in such environments has been limited in the past. If low-alloy steels can be employed in such environments, one can make full use of the advantageous properties of these steels, i.e., good weldability and high toughness.
(2) The wall thickness of steel parts can be decreased. As a result, the steel parts have improved thermal conductivity, leading to an improved thermal efficiency of a plant using the parts and reduced thermal fatigue, which the parts suffer when the operation of the plant is repeatedly started or stopped.
(3) The weight of steel parts can be reduced, resulting in a reduced size of a plant and reduced manufacturing costs.
Therefore, improvement in the strength of low-alloy steels provides significant practical benefits. The prior art techniques for increasing the strength of low-alloy steels have the problem that improvement in strength is accompanied by a loss of toughness.
For example, Cr--Mo steels such as T12 and T22 defined in ASTM and ASME get their high strength through a solid-solution strengthening effect of Mo and precipitation-strengthening effects of fine carbides of Cr, Fe, and Mo. However, the contribution of the effect of Mo is not significant and the above-described carbides are not effective in improving high-temperature strength, since the carbides are coarsened rapidly at high temperatures. A conceivable measure for improving the strength of these low-alloy steels is to increase the Mo content in order to increase the solid-solution strengthening effect. However, this measure is not practicable since the attainable improvement is not so large and the toughness, workability, and weldability of the steels are undesirably decreased.
The addition of precipitation-strengthening elements such as V, Nb, Ti, and B is effective in improving the strength of a low-alloy steel. On the other hand, they excessively harden the steels. Furthermore, particularly when precipitated in a matrix of ferritic phase, they cause a significant decrease in toughness. These elements also cause a significant loss of weldability. Therefore, the contents of these elements are limited in most applications.